With the recent increase in demand for semiconductor lasers in the fields of optical communications, laser printers, optical disk devices, and the like, semiconductor lasers of the GaAs type and the InP type, mainly, have been actively studied and developed. In the optical information processing field, a method of recording and reproducing information using light from an AlGaAs type semiconductor laser with a wavelength of 780 nm, especially, has been commercialized. Such a method has been widely used for compact disks and the like.
In recent years, optical disk devices with a larger memory capacity have been increasingly in demand. With this demand, shorter-wavelength lasers have been requested. An AlGaInP type semiconductor laser can oscillate in the red region of wavelengths of 630 to 690 nm, emitting light with the shortest wavelength among those obtained from semiconductor lasers practically available at present. This type of semiconductor laser is therefore highly expected to be a next-generation large-capacity light source for optical information recording, replacing the conventional AlGaAs type semiconductor laser. In general, when reproducing information from an optical disk, a semiconductor laser generates intensity noise due to return of light reflected from a disk surface and temperature change, inducing a signal read error. A laser with low intensity noise is therefore indispensable for a light source of an optical disk.
Conventionally, in order to reduce noise, a low-output AlGaAs type semiconductor laser for a reproduction-only device has a structure where saturable absorbers are intentionally formed on each side of a ridge stripe. With this structure, multiple longitudinal modes can be obtained. In the case where disturbances such as return light and temperature change arise when a laser is oscillated in a single longitudinal mode, oscillation in an adjacent longitudinal mode is started by a minute change in a gain peak, causing conflict with the oscillation in the original oscillating mode and thus leading to noise. When multiple longitudinal modes are used, the change in the intensity of each mode is averaged and is not influenced by the disturbances. Thus, stable low-noise characteristics can be obtained.
A method for obtaining further stable self-oscillation characteristics is disclosed in Japanese Laid-Open Publication No. 63-202083. In this publication, a self-oscillation type semiconductor laser has been realized by forming a layer which can absorb output light.
Japanese Laid-Open Publication No. 6-260716 reports that the characteristics have been improved by substantially equalizing the energy gaps of an active layer and an absorption layer. In particular, the energy gaps of a strained quantum well active layer and a strained quantum well saturable absorption layer are substantially equal to each other, so as to obtain good self-oscillation characteristics. A similar configuration is described in Japanese Laid-Open Publication No. 7-22695.
However, the inventors of the present invention have found that good self-oscillation characteristics are not obtained by only substantially equalizing the energy gaps of a saturable absorption layer and an active layer.
The present invention is aimed at providing a semiconductor laser having stable self-oscillation characteristics effective for noise reduction by examining the energy gap difference between a saturable absorption layer and an active layer, as well as a method for fabricating such a semiconductor laser and an optical disk device using such a semiconductor laser.